On the other side of the coin, as it were, a Greek exit from the euro zone would mean higher average credit quality for the countries still using the euro. But Steven Englander, head of G10 currency strategy for Citigroup, says that if any country, no matter how ailing, exits the euro zone, the perceived riskiness of the euro itself will rise.

"When the European Central Bank was put together, and almost for its entire history, there was never a sense that a country would drop out. There was never a Plan B," he told me. "So if one country goes, it sets the precedent that the market will ask questions about what other countries will go. Until those questions are resolved, it's bound to put downward pressure on the euro."

If investors are on guard for more potential dropouts, Englander says, we would see "downward pressure on the euro as a whole, and upward pressure on risk premia for countries considered a risk. That's not good for debt sustainability and not good for growth." In other words, a vicious cycle.

Englander told me he believes Greece will ultimately remain in the euro zone. But "Obviously, things have gotten worse" since European Union leaders announced their October deal, he says. "I think the euro is likely to be a weak currency."